When Bob and I got to talking about the "Darling" amp I was skeptical
about a 3/4 watt power amp. Yeah, I know it's the first watt that counts
and all, but I did not expect my speakers to be efficient enough to
really cut loose on 3/4 watt. I asked about push-pull possibilities, and
in fact built up a push-pull, IT-coupled amp. It didn't sound very good,
and I soon rewired it as a single-ended amp with the same IT coupling,
but with the 2 1626's in parallel - I had 4 sockets wired anyway. This
sounded quite a bit better, either in a conventional setup with the IT
as plate load for the driver, or as a parafeed coupled stage, with
resistors loading the plate and a cap coupling the driver to the
transformer. There was still a sense of some top end missing, as well as
some funky peaks in the response using the parafeed setup, so I went and
replaced the transformer with the cap that was in there anyway. This is
the topology I probably would have started with had I not tried a
push-pull setup first, and it is the method Bob used in his original
"Darling" circuit.
Note that I used 6SN7 to drive my amp, instead of 8532 as Bob used. I
was working with a junked chassis that had LOTS of octal sockets when I
was prototyping and I like the sound of 6SN7 in general. My amp has less
gain than Bob's as a result (but more power, huuh! huuh!) In my system,
with my preamp, this is not an issue.
I decided to set up the amp to roll off the bass for two reasons : I
suspected that the Hammond 125E transformers would be grateful for the
easier challenge, and I was intending to drive bass-challenged mini
monitors : any bass information that made it out to the speaker
terminals would die a quick and painless death there anyway. So I
implemented a 88Hz roll-off and added a powered subwoofer to pick up the
slack below the crossover point.
I think I am safe in saying I have a very loud ~1 1/2 WPC with this
arrangement!
"DC Double Darling" : Jeremy Epstein
Playing with the coupling schemes in my "Double Darling" amp got me
thinking. I had had a conversation with Don Garber, who manufactures an
interesting line of triode amps under the "Fi" brand name, and he
enthusiastically favors direct-coupled amps. One reason he gave me was
that they minimize the interaction between the LF cutofff of the circuit
itself and the LF phase-shift inherent in the transformer. (He uses the
same Hammond 125E in his least expensive amps, the WAY-cool "X" model
direct-coupled 2A3 amp. If I was going to BUY an amp, it would be from
Don, no question!)
Could the Darling be direct-coupled? I asked Bob what he thought, never
having built a direct-coupled amp myself and being under the impression
they were quite tricky to work out.
Well, as you can tell, Bob was also on the same track. (See his "DC
Darling" amp.) We talked about the various requirements, (primarily, a
400V power supply, or at least close to it, and high-wattage cathode R's
for the power tubes.) In April 1999 I got my version of this idea, the
"DC Double Darling," up and running. I really wouldn't say it was tricky
to work out : the same Ohm's law and power dissipation calculations one
would use to check voltage drops and watch out for spec'ing components
kept me on the right track here. The only problems I ran into were the
usual wiring errors, and replacing Bob's wacky shared cathode resistors
with the wrong values of individual resistors.
I'm still getting a feel for the differences between the cap-coupled and
DC "Darlings." The DC amp seems a tad faster, perhaps that's the 8532
driver, perhaps that's the DC coupling scheme. I will say that I like
both these amps quite a bit and so has everyone else who has listened to
them so far. They _definitely_ have the highest "bang-for-the-buck" of
anything I've ever built. Both my amps were built using power
transformers scavenged from trashed equipment, and the Hammond 125E
output transformers cost me less than $30 apiece, at retail. 1626's can
be bought for 5.50 at ANGELA INSTRUMENTS
and sometimes even cheaper :
that's a lot of 1626's for the price of one 2A3. I've learned a lot
going through all the iterations of the two amps and I've gotten some
pretty good sound as a result.
Betcha can't build just one!
Design Update - 5/11/99
This version of the DC Double Darling brings even more to the party than
the original iteration. In particular, imaging is much improved, higher,
wider, and with more front-to-back information : better all around. Also
fewer parts are used which gives you even MORE bang-for-the-buck.
No guarantees I don't change it again!
-j (ellenoler@earthlink.net)
No guarantees I don't change it again!